This invention relates to an inorganic electroluminescent (EL) panel, and more particularly, to a full color EL panel having light emitting layers for the three primaries RGB.
In the recent years, active research works have been made on thin-film EL devices as small or large-size, lightweight flat panel displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a dual insulated structure using thin-film insulating layers 2 and 4 as shown in FIG. 2. In FIG. 2, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrode-bearing substrate 1. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primaries of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescence center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primaries of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. Referring to the blue color among others, a relatively high luminance is achieved using SrS:Ce. However, its luminance is still short as the blue color for full-color display and its chromaticity is shifted toward green. There is a desire to have a better blue light-emitting layer.
To solve the above problem, thiogallate and thioaluminate base blue phosphors such as SrGa2S4:Ce, CaGa2S4:Ce, and BaAl2S4:Eu were developed as described in JP-A 7-122364, JP-A 8-134440, Communication Society Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys., Vol. 38 (1999), pp. L1291-1292. These thiogallate base phosphors are satisfactory in color purity, but suffer from a low luminance and especially, difficulty to form a thin film of uniform composition because of the multi-component composition. It is believed that thin films of quality are not obtainable because of poor crystallinity resulting from inconvenient composition control, formation of defects resulting from sulfur removal, and admittance of impurities; and these factors lead to a failure to increase the luminance. In particular, thioaluminate base phosphors are quite difficult to control their composition.
In order to develop practical full-color EL panels, phosphor materials capable of providing blue, green and red phosphors in a consistent manner and at a low cost are necessary. Since matrix materials of phosphor thin films and luminescence center materials individually have differing chemical or physical properties as described above, light-emitting performance differs depending on the identity of the phosphor thin film. Especially, the response speed and afterglow of light emission differ between different luminescence centers. To drive blue, green and red pixels, a burning method matching with each color is necessary.
Moreover, the EL spectra of the aforementioned blue, green and red EL phosphor thin films are all broad. When they are used in a full-color EL panel, RGB necessary as the panel must be cut out of the EL spectra of the EL phosphor thin films using filters. Use of filters complicates the manufacture process and, still worse, brings about a lowering of luminance. When RGB is taken out through filters, the luminance of blue, green and red EL phosphor thin films suffers a loss of 10 to 50% so that the luminance is reduced below the practically acceptable level.
To solve the above-discussed problem, there remains a need for red, green and blue phosphor thin film materials capable of emitting light of a sufficient color purity to eliminate a need for filters and at a high luminance, as well as an EL panel in which an identical luminescence center is used in red, green and blue phosphor thin films so that they have the same response speed and afterglow of light emission, allowing a common drive method to be used to drive blue, green and red pixels, without a need for a separate burning method matching with each color.
An object of the invention is to provide an EL panel comprising phosphor thin films eliminating a need for RGB phosphor filters, having a satisfactory color purity and best suited for driving RGB in full-color EL display.
This and other objects are attained by the present invention which provides an EL panel comprising EL phosphor thin films of three types which emit red, green and blue light, respectively, the EL phosphor thin films of three types commonly and essentially containing europium as a luminescence center.
In one preferred embodiment, the EL phosphor thin films of three types have the compositional formula:
AxByOzSw:R
wherein A is at least one element selected from the group consisting of Mg, Ca, Sr, Ba and rare earth elements, B is at least one element selected from the group consisting of Al, Ga and In, x is in the range of 0 to 5, y is in the range of 0 to 15, z is in the range of 0 to 30, w is in the range of 0 to 30, and R is an element serving as the luminescence center and essentially containing europium.
In a further preferred embodiment, the EL phosphor thin film which emits red light is made of a matrix material comprising an alkaline earth sulfide, the EL phosphor thin film which emits green light is made of a matrix material comprising an alkaline earth thiogallate, and the EL phosphor thin film which emits blue light is made of a matrix material comprising an alkaline earth thioaluminate. Typically, the alkaline earth sulfide is calcium sulfide; the alkaline earth thiogallate is strontium thiogallate; and the alkaline earth thioaluminate is barium thioaluminate.
In a further preferred embodiment, the EL phosphor thin films of three types which emit red, green and blue light, respectively, each comprise an oxysulfide obtained by incorporating oxygen in at least one compound selected from the group consisting of an alkaline earth sulfide, alkaline earth thioaluminate, alkaline earth thiogallate, and alkaline earth thioindate. The molar ratio of oxygen element to sulfur element in the oxysulfide, as expressed by O/(S+O), is in the range between 0.01 and 0.85.
In another embodiment, any one of the EL phosphor thin films of three types which emit red, green and blue light, respectively, is made of an oxide.